Conventionally, various optical fiber cables have been produced and used. Such optical fiber cables are produced by, for example, preparing a so-called optical fiber, which has a coating composed of a UV-curable resin or a thermosetting resin around the periphery of a glass optical fiber, and forming a sheath by providing a collective coating over this optical fiber, a pair of tension members and a support wire while positioning these members at specific locations. Examples of such an optical fiber cord or cable include those illustrated in FIG. 8, and are often referred to as “drop cables”.
A drop optical fiber cable 41 is formed by collectively coating an optical fiber 42, which is one or a plurality of single optical fibers, optical fiber ribbons and the like, tension members 43, 43′ made of steel wire, aramid resin and the like and a support wire 44 with a sheath 46 having a neck portion 45. To facilitate extraction of the internal optical fiber 42 etc. by breaking the sheath 46, in some cases a notch 47 is provided on the sheath 46. Further, for conventional optical fiber cables, to maintain flame retardance, it is common to use for the sheath material a flame retardant polyolefin having as a main component one or more resins selected from an ethylene-(meth)acrylic copolymer, an ethylene-alkyl(meth)acrylate copolymer and an ethylene-vinylacetate copolymer (e.g. Patent Documents 1 to 3).
However, when such optical fiber cables were laid overhead, over time their properties deteriorated for reasons which were unknown. Recently, it has finally been learned that the cause for this has been the egg-laying behavior of cicadas, which emerge during the summer season.
Specifically, the cause is the behavior of cicadas, which mistake the optical fiber cables laid overhead for the trunk or branches of a tree, cutting through the sheath with their ovipositor and laying eggs inside. If the ovipositor is inserted into the sheath in this manner, the optical fiber cable is damaged by the ovipositor, or moisture such as rain water tends to infiltrate into the cable interior from the opened holes. If moisture infiltrates into the cable in this manner, the risk of increased transmission loss due to this moisture dramatically increases.
Accordingly, as described in Patent Document 4, an optical fiber cable has been proposed in which a protective tape is arranged inside or on the outer surface of the sheath so as to at least partially surround the optical fiber which is coated by the sheath. If such an optical fiber cable is used, even should a cicada cut through the sheath with its ovipositor, the tip of the ovipositor is blocked by the protective tape and thus does not reach as far as the internal optical fiber. As a result, the above-described risks, specifically, the risks of optical fiber cable damage and increased transmission loss due to infiltrated moisture can be reduced.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-337255
Patent Document 2: Japanese Patent Application Laid-Open No. 2001-166188
Patent Document 3: Japanese Patent Application Laid-Open No. 2001-208942
Patent Document 4: Japanese Patent Application Laid-Open No. 2006-313314